High intensity, pulsed thermal neutron source

ABSTRACT

This invention relates to a high intensity, pulsed thermal neutron source comprising a neutron-producing source which emits pulses of fast neutrons, a moderator block adjacent to the fast neutron source, a reflector block which encases the fast neutron source and the moderator block and has a thermal neutron exit port extending therethrough from the moderator block, and a neutron energy-dependent decoupling reflector liner covering the interior surfaces of the thermal neutron exit port and surrounding all surfaces of the moderator block except the surface viewed by the thermal neutron exit port.

United States Patent 1 [111 3,778,627

Carpenter Dec. 11, 1973 [54] HIGH INTENSITY, PULSED THERMAL 3,345,51510/1967 Adachi 250/499 NEUTRON SOURCE 3,349,001 10/1967 Stanton....250/499 U Inventor: T Carpenter Ann Arbor Primary Examiner-James W.Lawrence c Assistant Examiner-T. N. Grigsby [7 3] Assignee: The UnitedStates of America as A t0mey-Roland A. Anderson represented by theUnited States Atomic Energy Commission, [57] ABSTRACT washmgton Thisinvention relates to a high intensity, pulsed ther- [22] Filed: Apr. 17,1973 mal neutron source comprising a neutron-producing source whichemits pulses of fast neutrons, a moderator block adjacent to the fastneutron source, a reflector block which encases the fast neutron sourceand [2i 1 App No.: 351,893

[52] U.S. Cl 250/499, 250/502, 250/518 th m rat block and as a th rmal nutron xit [51] Int. Cl 621g 3/04 p ex n ing h r hr gh from h moder tor[58] Field of Search 250/499, 500, 501, block, n a r n n rgypen n decpling r 250/502, 518, 393 flector liner covering the interior surfacesof the thermal neutron exit port and surrounding all surfaces of [56]References Cited the moderator block except the surface viewed by theUNITED STATES PATENTS thermal neutron exit port. 2,253,035 8/1941Kallmann .Q 250/499 14 Claims, 4 Drawing Figures III/1 I]!!! III]PATENTED DEC 1 1 I975 SHEET 1B? 2 m w -lgii M 1 m mi my 0.; 6e) ProtonsSHEET 2 BF 2 PATENIEU DEC 1 1 I975 Flaw- HIGIIA INTENSITY, PULSEDTHERMAL NEUTRON SOURCE CONTRACTUAL ORIGIN OF THE INVENTION The inventiondescribed herein was made in the course of, or under, a contract withthe UNITED STATES ATOMIC ENERGY COMMISSION.

BACKGROUND OF THE INVENTION This invention relates generally to neutronsources and. more particularly to pulsed thermal neutron sources.Specifically, this invention is concerned with a novel apparatus forslowing down fast neutrons in such amanneras to create highly intensepulses of thermal neutrons. 7'

Use of neutrons in research and development has grown immensely inimportance, and applications of both fast and thermal neutrons haveranged from pure to applied research as well as in such directapplicationsaslwith nuclearreactorsand nuclear medicine. In order toobtain beams of neutrons, both fast and thermal as well as steady andpulsed, nuclear reactors have been built in the past specifically forthis purpose. Some examples of such facilitiesare the High Flux IsotopeReactor at Oak Ridge National Laboratory which provides a thermalneutron beam source flux of 1.0 X 10" neutrons (n)/cm .-sec, the HighFlux Beam Reactor :at Brookhaven National Laboratory which produces athermal neutron beam source flux of5.0 X 10 nlcm -sec, and theGerman-Frenchhigh flux reactorat the .von Laue-Langevin Institute atGrenoble which provides a thermal neutron beam source flux of 1.0 X 10nlcm -sec. Unfortunately, advances in the neutron flux provided by suchreactors have slowed as the reactor designs have approached their limitsimposed by heat transfer and operating costs.

A new generation of neutron sources, however, is being developed wherebythe costs. of building, operating and maintaininga steady-state nuclearreactor are not required. These sources utilize pulsed reactors orvarious types of accelerators to produce both fast and thermal neutronbeams. A few examples of these are the SORA pulsed reactorwhich would beexpectedto produce a neutron beam source flux of 1.0 X nlcm -sec, theBPFR atBrookhaven National Laboratory whichwould be expected toprovide aneutron flux greaterthan 10" nlcm -sec, the Canadian Intense NeutronGenerator whichwouldbe expected to produce a steadybneutron flux of l X10" n/cm'.-sec, and the WNRF at Los Alamoswhere a neutron flux of. about2 X 10 nlcm -sec isexpected. The. inherent qualities of such neutronsources as compared with steady+state nuclear reactors make them highlydesirable. Furthermore, the more intense the neutron flux obtainablefrom such a neutron source is, the more desirable such a source becomes.

The inventor has developed a new approach. to the design of pulsedmoderatorswhich not. only. eliminates the need for a nuclear. reactorwith its associatedhigh costs of constructionandoperation, but also iscapable of obtaining beams of; pulsed thermal neutrons of extremelyhigh; intensity. Through the use of a fast neutron source and a novelarrangement of a moderator, a reflector and a neutronenergy-dependentdecoupling reflector liner, this apparatus iscapable ofproviding 60 pulses/sec with the peak thermal neutron fluxachievableatthemaxirnum of each pulse being 1.6 X 1 0" nlcm -sec. Thismakes the present invention one of the most intense pulsed thermalneutron sources in the world.

Therefore, it is one object of the present invention to provide a pulsedthermal neutron beam source capable of producing a peakflux of 1.6 X 10nlcm -sec.

It is another object of the present invention to provide a novelapparatus for slowing down fast neutrons so as to produce highly intensepulses of thermal neutrons.

Further objects and advantages of the invention will be apparent fromthe following detailed description of the invention.

SUMMARY OF THE INVENTION A moderator block constructed from ahydrogenous material such as polyethylene is located adjacent to apulsed fast neutron source. A reflector block constructed from amaterial such as beryllium completely encases the fast neutron sourceand the moderator block except for a thermal neutron exit port whichextends through the reflector block from the moderator. The interiorsurfaces of this port and all surfaces of the moderator except thatsurface viewed by the port are covered with a neutron energy-dependentdecoupling reflector liner made from a materialsuch as cadmium.

When this apparatus is in operation, pulses of fast neutrons radiateoutwardly from the fast neutron source. A portion of the fast neutronsfrom each pulse strike the moderator and become thermalized therein, themoderator emitting neutrons in the thermal energy ranges starting about3 usec after the fast neutrons from each pulse strike the moderator. Thepurpose of the reflector block is to enhance the intensity of theemerging pulsed thermal neutron beam by partially thermalizing thosefast neutrons which do not initially enter the moderator and deflect aportion thereof back into the moderator. In addition, the reflectorblock partially thermalizes and reflects back to the moderator fastneutrons which pass unthermalized through the moderator. Since a shortpulse width of only 30 usec for each emerging thermal neutron pulse isdesired, these fast neutrons which are partially thermalized in thereflector block and then deflected back into the moderator must be sodeflected within about 15 usec. Otherwise, the resultant thermal neutronpulse width willbe considerably greater than 30 usec.

Since neutrons thermalized in the reflector block have long lifetimestherein inasmuch as they can remain in the reflector block for a matterof milliseconds, and since neutrons thermalized within the moderatorhave a very short lifetime of about 30 usec, a neutronenergy-dependentdecoupling reflector liner is placed, as describedabove, around the moderator. This reflector liner decouples thereflector block from the moderator in that only neutrons having anenergy at or greater than a certain threshold or decoupling energy aretransmitted. through the reflector. liner, while neutrons havingenergies below this decoupling energy level are captured and absorbed bythe reflector liner, thereby, preventing them. from entering themoderator block. Thisdecoupling energy is chosen so that the thereforethe thermal neutron pulse width, is about 30 usec. Therefore, neutronswhich have been slowed down in the reflector block for a sufficientperiod of time so that their energies are less than the decouplingenergy before being reflected back to the moderator cannot pass throughthe reflector liner and into the moderator, thereby insuring that theresulting thermal neutron pulses from the moderator will be littlebroadened by the reflected neutrons and will have the desired shortpulse width of about 30 usec. Without such an energy-dependentdecoupling reflector liner, neutrons having long lifetimes in thereflector block would continuously be deflected into the moderator,resulting in either very long thermal neutron pulse widths or just asteady thermal neutron beam lacking individual pulses. However, thoseneutrons which are partially thermalized in the reflector block anddeflected back to the moderator within psec not only create shortthermal pulse widths but also complement and increase the resultingthermal neutron pulses, thereby enabling the present invention toachieve high-intensity pulses of thermal neutrons.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a horizontal cross-sectionalview of a source-moderator-reflector arrangement with beam port inaccordance with the present invention.

FIG. 2 is a three-dimensional outer view of the embodiment illustratedin FIG. 1.

FIG. 3 is a three-dimensional view of one particular arrangement of thefast neutron source, moderator block and reflector liner.

FIG. 4 is a horizontal cross-sectional view of a facility incorporatingthe present invention.

DETAILED DESCRIPTION OF THE INVENTION Referring first to the embodimentshown in FIG. 1, apparatus 10 consists of a pulsed fast neutron source12 which is disposed within a source well 14. Located adjacent to fastneutron source 12 is moderator block 16 composed of a hydrogenousmaterial such as polyethylene. Surrounding and encasing source 12 andmoderator 16 is reflector block 18 composed of a material such asberyllium. Reflector block 18 serves to return into moderator 16 fastneutrons from source 12 which would otherwise leak out of moderator 16while slowing down, and to deflect into moderator l6 neutrons whichwould not collide if the reflector block were not present. Thermalneutron exit port 20 extends through reflector block 18 to moderator l6and serves as the exit passageway from moderator 16 for thehighintensity thermal neutron pulses. Neutron energydependent decouplingreflector liner 22 covers the interior surfaces of port 20 and allsurfaces of moderator 16 except surface 24 which is viewed by port 20.Reflector liner 22 insures that only those neutrons which have beenpartially thermalized in reflector block 18 to no lower than thedecoupling energy can be returned into moderator 16. Also, the portionof liner 22 which covers the surfaces of port 20 serves to capture andabsorb any thermal neutrons exiting from moderator 16 which would strikethe surfaces of port 20, resulting thereby in an essentially parallelbeam of pulsed thermal neutrons exiting from apparatus 10.

FIG. 2 illustrates a three-dimensional view of the exterior of apparatus10. As indicated previously, reflector block 18 entirely surrounds andencases the fast neutron source and moderator block except for thermalneutron exit port 20.-Also indicated in FIG. 2 is a second port which isproton beam entry port 26. While the present invention is not limited toany particular type of pulsed fast neutron source, one embodiment of theinvention, discussed in more detail below, utilizes a spallation targetwhich emits fast neutrons upon interaction with an energetic protonbeam. Consequently, in this particular embodiment a proton beam entryport 26 must extend through reflector block 18 to the spallation target,the fast neutron source.

The reflector block is constructed from a material which will slow downfast neutrons which would otherwise escape and which has a low capturecross section so as not to absorb neutrons to any appreciable extent.Preferably, the material should have a high density of nuclei with lowatomic mass, so that mean slowing down times and pulse widths are assmall as possible. In addition, the mean slowing down distance in thereflector block material should be neither too large nor too small. Ifthis distance is too large as compared to the size of the moderatorblock, then the reflector block's effectiveness in reflecting neutronsback into the moderator is reduced. If this distance is too small, thenits effectiveness in deflecting neutrons into the moderator whichotherwise would have missed is reduced. Therefore, beryllium is thepreferred material for the reflector block, beryllium having anadditional advantage in that neutrons produced by (n, 2n) and ('y, n)reactions add to the thermal neutron flux. Beryllium is also relativelyeasy to handle. Other materials which can be utilized as the reflectorblock material include heavy water (D 0), high deuteron densitymaterials such as ND;,, and possibly heavy metal deuterides such astitanium deuteride.

The moderator block must be capable of thermalizing fast neutrons in asmall number of elastic collisions resulting in short neutron lifetimestherein. Therefore, a hydrogenous material with a high density offreelymoving protons is preferred. Materials which would function'well'as the moderator block are polyethylene, water, solid or liquidmethane or ammonia, liquid hydrogen,and heavy metal hydrides. However,some of these materials such as liquid methane and liquid hydrogenpresent additional problems and difflculties if they are to be utilizedas the moderator. Therefore, polyethylene is the preferred material,especially for testing purposes.

The neutron energy-dependent decoupling reflector liner must comprise amaterial which has a capture cross section such that it will onlytransmit neutrons at or above a specified decoupling energy aspreviously explained. Cadmium and possibly gadolinium are good materialsfor the reflector liner. Therefore, if beryllium is utilized as thereflector block material and cadmium as the energy-dependent decouplingreflector liner, the decoupling energy is about 0.4 eV, and theslowingdown time of fast neutrons to 0.4 eV in beryllium is about 15usec. A decoupling energy lower than 0.4 eV would allow neutrons to passinto the moderator which have been in the reflector block for longerthan 15 usec and would thereby increase the thermal neutron pulse widthto more than 30 usec. If liquid D 0 is utilized as the reflector blockmaterial, the decoupling energy could be lower than 0.4 eV resulting inan increase in the moderator output, for slowing-down times in D 0 aresmaller than in beryllium. For instance, to slow down fast neutrons to0.4 eV takes only about 8 usec in D 0. However, a faster slowing-downtime in the reflector block means fewer collisions of the neutrons inthe reflector block and therefore fewer chances for deflection to themoderator unless the decoupling energy of the reflector liner isdecreased. The reflector liner must be closely adjacent to the moderatorto avoid lengthening the pulse width due to the finite flight times ofneutrons between the reflector liner and the moderator.

Conveniently, as shown in FIG. 1, the path of the thermal neutronsemerging from the moderator is essentially at right angles to a lineintersecting the fast .neutron source and the moderator. This is thepreferred arrangement wherein the fast neutron source, moderator andthermal neutron exit port are not directlyaligned. Such an arrangementis advantageous in that whatever views the uncovered moderator surfacethrough the exit port will not be in a direct line with the fast neutronsource. In addition, fast neutrons which pass unthermalized through themoderator will strike the reflectorblock and possibly be deflected backinto the moderator. If the fast neutron source, moderator and exitportwere all aligned in a straight line, however, such unthermalized fastneutrons would pass out of the moderator and directly into the thermalneutron exit port. Such a situation would be quite undesirable.

As stipulated previously, the present invention is not limited to anyparticular type of pulsed fast neutron SOUI'CB.HOWV6T, FIG. 3illustrates a particular spallation neutron source-moderator arrangementwhich can be utilized in the specific embodiment explainedbelow.-Spallation target 28 comprises a 15 cm by 10 cm diameter cylinderof depleted U located 2.5 cm from a 10 cmX 10 cm X 7.65 cm polyethylenemoderator block .30. All surfaces of moderator block 30 are covered by acadmium neutron energy-dependent decoupling .reflector liner 32 exceptsurface 34 which is viewed by the thermal neutron exit port (not shown).The pulsed thermal neutron beam exits from moderator 30 by way ofsurface 34 and is indicated by the arrow and the symbol I whichrepresents the thermal neutron ,beam current per unit solid angle. The Uspallation target 28 is impinged by pulses of 0.5 GeV protons from theinjector booster accelerator of the ZeroGradient Synchrotron (ZGS)proton accelerator at Argonne National Laboratory which operates ataboutS X l proton/pulse at a repetition rate of 60 Hertz, the pulsewidth being about 180 nsec maximum. With about 6/7 of all acceleratorbeam pulses being available for impingement on the U spallation target,this: results in an average fast neutron production of abouts X 10 nlsecwith the peak fast neutron production being about 6.5 X nlsec.

While U is utilized as the spallation target material in the aboveexample, any spallation target material of heavy nuclei which functionsto produce pulses of fast neutrons. of about l0 nlsec or more at about100 Hertz can be utilized in the present invention, for examplelead-bismuth eutectic. In addition, nonspallation types of fastineutronsourcescan be utilized with the present invention, such asBremsstrahlung photoneutron sources, pulsed deuterium-tritium fusionneutron sources or other charged particle neutron sources.

ter of facility 36. Adjacent to fast neutron source 38 is moderatorblock 40, while reflector block 42 encases fast neutron source 38 andmoderator 40. Completely surrounding reflector block 42 is shielding 44and shielding 46 which may be constructed from any appropriate shieldingmaterial known to the art. Passing through shieldings 44 and 46 andthrough reflector block 42 are a plurality of thermal neutron exit ports48. Neutron energy-dependent decoupling reflector liner 50 covers theinterior surfaces of ports 48 and all surfaces of moderator block 40except surface 52 which is viewed by exit ports 48. In addition to beinga pulsed thermal neutron source facility, facility 36 can also beutilized as a source of fast neutrons by incorporating a fast neutronexit port 54 in facility 36, port 54 passing through shieldings 44 and46 and through reflector block 42 so as to view fast neutron source 38.It should be noted that if fast neutron source 38 is a spallation targetsource, the proton beam entry port would be vertical and therefore isnot shown in FIG. 4. In addition to the illustration shown in FIG. 4,several moderator blocks could be served by the fast neutron source 38with each moderator block being viewed by several thermal neutron exitports. As can be seen from FIG. 4, the variety of designs andconfigurations of facilities which could incorporate and utilize thepresent invention are numerous, and therefore the present invention isnot limited to such.

Turning back to FIGS. 1 and 2, a specific embodiment for steady-statetesting purposes was constructed in accordance therewith. The moderatorwas a 4 inch x 4 inch X 2 inch polyethylene block with a density of 0.91gm/cm. The pulse full width at half maximum for 0.05 eV thermal neutronsexiting from the moderator would be about 30 ,usec. A 4 inch X 4 inchsource well was chosen to simulate the U spallation source describedabove for FIG. 3. The 4 inch X 4 inch moderator was chosen since themean slowing-down distance for fast neutrons in polyethylene is about 10cm as well as the fact that the range over which the fast neutrons areproduced in the fast neutron source is also about 10 cm. The beam portwas selected to be 4 inch X 4 inch square due to the choice of themoderator size.

The reflector block was constructed from beryllium and had the outerdimensions of I8 inches X 18 inches X 16 inches. The asymmetric shapewas chosen and the dimensions made smaller than optimally desirable inorder to minimize the mass of material to be handled, even though theroot-mean-square slowing-down distance from 2 MeV to 0.4 eV for fastneutrons in beryllium is about 25 cm. The reflector block was surroundedon five faces with 6 inches of Benelex, a tradenamed material whichcomprises a high-density woodbased composite, and rested above 3 inchesof Benelex on a concrete floor, resulting thereby in insignificant roomscatter and efficient reflection of neutrons leaking from the reflectorblock.

The neutron energy-dependent decoupling reflector liner was 0.020 inchcadmium, resulting in as near an optimal choice of apparatus materialsas could be easily constructed. As discussed above, the berylliumreflector block cadmium reflector liner combination resulted in adecoupling energy of about 0.4 eV, for the slowing-down time to 0.4 eVfor fast neutrons in beryllium is about 15 nsec.

In lieu of utilizing a U" spallation source as illustrated in FIG. 3 anddescribed above, unstable isotopes which had been previously accuratelycalibrated by A. DeVolpe, Inorganic ChemistryLetters, 5, 128 (1968),were utilized as low-intensity fast neutron sources in the measurementsfor testing and evaluating the above embodiment of the presentinvention. Cf with a fission-neutron spectrum and Am -Be with the harderspectrum characteristic of (a, n) sources, were utilized, for they wouldalso enable the determination of the effects of source spectralvariation.

Neutron fluxes were measured utilizing the very sensitive solid statetrack recording (SSTR) method of R. Gold, et al., Nuclear Science andEngineering 34, 13 (1968), using asymptotically thick 93.1 percentenriched uranium fission foils and mica track recorders, for it wasdetermined that steady-state measurements are sufficient to determinethe intensity for pulsed moderators for which the pulse shape and energydistribution of the sources are known. Neutrons were incident onlythrough the track recorder side so that no significant self-shieldingoccurred. Track densities were to 10 tracks/cm and were determined bymanual counting, with errors of the area of 5 percent assuming Poissonstatistics in the numbers of tracks counted.

Both bare and cadmium-covered detectors were irradiated, and two classesof measurements were made. The flux on the moderator surface wasmeasured by detectors placed directly against the surface. Detectorsplaced 36 cm and 100 cm from the moderator, well shielded againstroom-scattered neutrons, were used to measure the beam current per unitsolid angle. The net sub-cadmium track densities were converted tothermal flux, assuming a spectrum-averaged fission cross section of 439barns/U which is characteristic of a spectrum whose temperature, 365I(.,is that expected for this polyethylene moderator.

The thermal neutron beam current per unit solid angle, I was determinedfrom the track densities of detectors irradiated at a distance down thebeam from the thermal neutron exit port. In addition, the averagethermal neutron flux, di on the viewed moderator surface, A was relatedto the thermal-neutron beam current per unit solid angle in accordancewith equation (1 Th ave. 4,29 (In/ m The results of the measurements andtests were consistent with the previous calibrations for Cf and Am" Y"-Be. The flux per unit source was about the same for both the Cf andAm-Be sources, indicating only mild dependence of the yield on theparticular source spectrum. The flux distribution appeared to besomewhat tilted toward the source in the case of the C? source but wasvirtually symmetric about the viewed moderator surfaces centerline inthe case of the AM" -Be source.

The cadmium ratio, defined as the track density for bare SSTR/trackdensity for cadmium-covered SSTR, observed at the moderator faceappeared to be characteristic of the emerging thermal neutron beam andwas observed to be influenced strongly by epicadmium neutrons from thethermal neutron exit port walls. The thermal neutron beam current perunit solid angle seemed to be a little greater than would be expected asseen when the average thermal neutron surface flux calculated from thebeam current in accordance with equation (1) was compared with themeasured flux on the viewed moderator surface.

Summarizing the results of the measurements, then,

the maximum thermal neutron flux at the viewed moderator surface takenas the average of two measurements was 3.2 X 10" thermal neutrons (n)/cm /fast neutron (n;), and the thermal neutron beam current per unitsolid angle, averaging three measurements, was 4.1 X 10' n lsteradianlnThe pulse width at 0.05 eV for the polyethylene moderator was 30 usec.

The particular embodiment described above having been tested with knownneutron sources and found to be completely operable, a U spallationsource, illustrated in FIG. 3 and described above, is placed in thesource well. Pulses of 0.5 GeV protons are impinged upon the U target,creating fast-neutron pulses of about 9.8 X 10 n IpuIse. With theinvention operating as explained above, a maximum peak thermal neutronflux of about 1.6 X l0 n /cm sec is achieved each time the U spallationtarget receives a proton pulse, the pulse width for 0.05 eV thermalneutrons being about 30 [1.866, and the time average thermal neutronflux is about 2 X 10 nlcm -sec. This makes the present invention one ofthe most intense pulsed thermal neutron sources in the world. 7

Generally, the peak flux of this embodiment would remain roughlyunchanged if the moderator were heterogeneously poisoned to reduce thepulse width. In addition, the beryllium reflector block with theenergydecoupling reflector liner provides about a 5 or 10 timesenhancement of the peak thermal neutron flux compared to that achievablewith a bare moderator.

It should be noted that the reflector block moderator reflector linercombination has not been optimized. Therefore, further enhancement ofthe peak thermal neutron flux is possible. On the other hand, thiscombination does not incorporate any design requirements for cooling dueto heat production resulting from thermal power released in the targetfrom incident proton energy, fissions and other reactions, or allowancesto account for thermoelastric shock effect. Such design considerationsfor cooling will most likely detract from the peak thermal neutron flux.However, it is felt that the increase in peak flux resulting from designoptimizations would compensate for the required practical changes forcooling, leaving the peak thermal neutron flux of the present inventionat about 1.6 X 10" n lcm -sec.

It should be further noted that, from the measurements made above, thepeak thermal leakage beam current per unit solid angle, integrated overthe viewed moderator surface, for 0.05 eV-neutrons is about 1.34 X 10" n/steradian-sec, and this quantity is what is required for estimates ofperformances in beam application.

' It will be understood that the invention is not to be limited to thedetails and specific embodiments given herein but that it may bemodified within the scope and spirit of the appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

l. A high-intensity, pulsed thermal neutron source comprising aneutron-producing source which emits pulses of fast neutrons; amoderator block adjacent to the fast neutron source; a reflector blockencasing said fast neutron source and said moderator block, saidreflector block having a thermal neutron exit port extendingtherethrough from said moderator block; and a neutron-energy-dependentdecoupling reflector liner covering the interior surfaces of said portand all surfaces of said moderator block except the moderator surfaceviewed by said thermal neutron exit port.

2. The thermal neutron source according to claim 1 wherein saidneutron-energy-dependent decoupling reflector liner comprises a materialwhich will capture and absorb neutrons having energies below thedecoupling energy and transmit neutrons having energies at or above saiddecoupling energy, said decoupling energy being related to that energyto which said reflector block will slow down said fast neutrons in atime less than the response time of said moderator block.

3. The thermal neutron source according to claim 2 wherein saiddecoupling energy is that energy to which said reflector block will slowdown said fast neutrons in 15 usec.

4. The thermal neutron source according to claim 1 wherein saidreflector block comprises a material having a high density of deuterons.

5. The thermal neutron source according to claim 1 wherein saidreflector block comprises a material selected from the group consistingof beryllium, heavy water and heavy metal deuterides.

6. The thermal neutron source according to claim 1 wherein saidmoderator block comprises a material having a high density of protons.

7. The thermal neutron source according to claim 6 wherein saidmoderator block is selected from the group consisting of polyethylene,water, non-gaseous methane, non-gaseous ammonia, liquid hydrogen andheavy metal hydrides.

8. The thermal neutron source according to claim 1 wherein saidneutron-energy-dependent decoupling reflector liner comprises a materialhaving a high capture cross section for absorption of low-energyneutrons.

9. The thermal neutron source according to claim 8 wherein saidneutron-energy-dependent decoupling reflector liner is selected from thegroup consisting of cadmium and gadolinium.

10. A pulsed thermal neutron source capable of emitting high-intensitythermal neutron pulses of about 30 usec pulse width comprising aneutron-producing source which emits pulses of fast neutrons; apolyethylene moderator block adjacent to the fast neutron source; aberyllium reflector block encasing said fast neutron source and saidpolyethylene moderator block, said beryllium reflector block having athermal neutron exit port extending therethrough from said polyethylenemoderator block; and a cadmium neutron-energydependent decouplingreflector liner covering the interior surfaces of said port and allsurfaces of said polyethylene moderator block except the moderatorsurface viewed by said thermal neutron exit port.

11. The thermal neutron source according to claim 10 wherein said fastneutron source comprises a spallation target material capable ofemitting pulses of fast neutrons upon interaction with energetic pulsedproton beams, and wherein said beryllium reflector block has a protonbeam entry port extending therethrough from said spallation targetmaterial.

12. The thermal neutron source according to claim 11 wherein saidspallation target material comprises a material selected from the groupconsisting of U and lead-bismuth eutectic.

13. The thermal neutron source according to claim 12 wherein said fastneutron source comprises U impinged upon by 0.5 GeV pulsed proton beams.

14. The thermal neutron source according to claim 10 wherein said fastneutron source, said moderator block, said neutron-energy-dependentdecoupling reflector liner and said thermal neutron exit port arearranged so that the path of said thermal neutron pulses emerging fromsaid moderator block is essentially at right angles to a lineintersecting said fast neutron source and said moderator block.

t t 1i

1. A high-intensity, pulsed thermal neutron source comprising aneutron-producing source which emits pulses of fast neutrons; amoderator block adjacent to the fast neutron source; a reflector blockencasing said fast neutron source and said moderator block, saidreflector block having a thermal neutron exit port extendingtherethrough from said moderator block; and a neutron-energydependentdecoupling reflector liner covering the interior surfaces of said portand all surfaces of said moderator block except the moderator surfaceviewed by said thermal neutron exit port.
 2. The thermal neutron sourceaccording to claim 1 wherein said neutron-energy-dependent decouplingreflector liner comprises a material which will capture and absorbneutrons having energies below the decoupling energy and transmitneutrons having energies at or above said decoupling energy, saiddecoupling energy being related to that energy to which said reflectorblock will slow down said fast neutrons in a time less than the responsetime of said moderator block.
 3. The thermal neutron source according toclaim 2 wherein said decoupling energy is that energy to which saidreflector block will slow down said fast neutrons in 15 Mu sec.
 4. Thethermal neutron source according to claim 1 wherein said reflector blockcomprises a material having a high density of deuterons.
 5. The thermalneutron source according to claim 1 wherein said reflector blockcomprises a material selected from the group consisting of beryllium,heavy water and heavy metal deuterides.
 6. The thermal neutron sourceaccording to claim 1 wherein said moderator block comprises a materialhaving a high density of protons.
 7. The thermal neutron sourceaccording to claim 6 wherein said moderator block is selected from thegroup consisting of polyethylene, water, non-gaseous methane,non-gaseous ammonia, liquid hydrogen and heavy metal hydrides.
 8. Thethermal neutron source according to claim 1 wherein saidneutron-energy-dependent decoupling reflector liner comprises a materialhaving a high capture cross section for absorption of low-energyneutrons.
 9. The thermal neutron source according to claim 8 whereinsaid neutron-energy-dependent decoupling reflector liner is selectedfrom the group consisting of cadmium and gadolinium.
 10. A pulsedthermal neutron source capable of emitting high-intensity thermalneutron pulses of about 30 Mu SEC pulse width comprising aneutron-producing source which emits pulses of fast neutrons; apolyethylene moderator block adjacent to the fast neutron source; aberyllium reflector block encasing said fast neutron source and saidpolyethylene moderator block, said beryllium reflector block having athermal neutron exit port extending therethrough from said polyethylenemoderator block; and a cadmium neutron-energy-dependent decouplingreflector liner covering the interior surfaces of said port and allsurfaces of said polyethylene moderator block except the moderatorsurface viewed by said thermal neutron exit port.
 11. The thermalneutron source according to claim 10 wherein said fast neutron sourcecomprises a spallation target material capable of emitting pulses offast neutrons upon interaction with energetic pulsed proton beams, andwherein said beryllium reflector block has a proton beam entry portextending therethrough from said spallation target material.
 12. Thethermal neutron source according to claim 11 wherein said spallationtarget material comprises a material selected from the group consistingof U238 and lead-bismuth eutectic.
 13. The thermal neutron sourceaccording to claim 12 wherein said fast neutron source comprises U238impinged upon by 0.5 GeV pulsed proton beams.
 14. The thermal neutronsource according to claim 10 wherein said fast neutron source, saidmoderator block, said neutron-energy-dependent decoupling reflectorliner and said thermal neutron exit port are arranged so that the pathof said thermal neutron pulses emerging from said moderator block isessentially at right angles to a line intersecting said fast neutronsource and said moderator block.